1. Field of the Invention
The present invention relates to a pressure detector of the single-diaphragm electrostatic capacitance type for measuring pressure on the basis of the electrostatic capacitance formed between a single diaphragm displaceable in response to pressure and each of fixed electrodes disposed at the opposite sides of the diaphragm, in which the influence of the electrostatic capacitance at a peripheral edge junction portion between the diaphragm and each of the fixed electrodes is eliminated so as to improve linearity in the detection signal. The present invention also relates to a pressure detector of the twin-diaphragm electrostatic capacitance type, in which the influence of the electrostatic capacitances at the respective peripheral edge portions between each of the diaphragms and the fixed electrodes associated thereto is eliminated so as to improve linearity in the detection signal.
2. Description of the Related Art
Such a pressure detector of the single-diaphragm electrostatic capacitance type for measuring pressure on the basis of the electrostatic capacitance formed between a single diaphragm displaceable in response to pressure and each of fixed electrodes disposed at the opposite sides of the diaphragm is disclosed, for example, in U.S. Pat. No. 2,999,386, entitled "HIGH PRECISION DIAPHRAGM TYPE INSTRUMENTS."
FIG. 13 is a sectional view illustrating the configuration of a main portion of such a conventional single-diaphragm electrostatic capacitance type pressure detector. In FIG. 13, a diaphragm 69 of silicon has fixed electrodes 61 and 62 joined thereto through glass junction portions 67 and 68. An air gap 65 is formed between diaphragm 69 and fixed electrode 61, and an air gap 66 is formed between diaphragm 69 and fixed electrode 62. A pressure leading-in hole 63 is formed in fixed electrode 61 for leading pressure P1 into the air gap 65, and a pressure leading-in hole 64 is formed in the fixed electrode 62 for leading pressure P2 into the air gap 66.
A first capacitor consists of diaphragm 69 and fixed electrode 61, and the electrostatic capacitance C61 of this capacitor is taken out through lead pins A1 and A3. A second capacitor is constituted by the diaphragm 69 and fixed electrode 62, and the electrostatic capacitance C62 of this capacitor is taken out through lead pins A2 and A3. The reference numerals 61a, 62a and 69a represent electric conductor plates. When the pressures P1 and P2 act upon the diaphragm 69, the diaphragm is displaced corresponding to the differential pressure (P2-P1), and the electrostatic capacitances C61 and C62 vary corresponding to the displacement of diaphragm 69. It is possible to measure the differential pressure on the basis of this displacement.
Generally, the pressure detector shown in FIG. 13 is placed in a hermetically sealed housing with two seal diaphragms receiving the pressures P1 and P2, and an incompressible fluid for pressure transmission. For example, when silicone oil is enclosed in the housing, the air gaps 65 and 66 and the pressure leading-in holes 63 and 64 are filled with silicone oil.
In the conventional single-diaphragm type detector described above, two capacitors are actually formed between diaphragm 69 and fixed electrode 61. One capacitor is constituted by diaphragm 69 and fixed electrode 61 with the air gap 65 interposed in between. Its electrostatic capacitance is given by Co/(1-.DELTA./D), in which Co represents the electrostatic capacitance when the differential pressure (P2-P1) is zero, .DELTA. represents the displacement of the diaphragm 69 in the right direction due to the differential pressure (P2-P1), and d represents the thickness of the air gap 65 when the differential pressure (P2-P1) is zero. It is generally assumed that P2 is greater than P1.
The other capacitor consists of diaphragm 69 and fixed electrode 61 with the glass junction 67 in between. Its electrostatic capacitance C67 is independent of the displacement of diaphragm 69, and thus has a harmful effect on the measurement.
Two capacitors are also formed between diaphragm 69 and fixed electrode 62. One capacitor is formed by the diaphragm 69 and fixed electrode 62 with the air gap 66 interposed in between, its electrostatic capacitance being given by Co/(1+.DELTA./d). The other capacitor is formed by diaphragm 69 and fixed electrode 61 with the glass junction portion 67 interposed in between, its electrostatic capacitance C68 is independent of the displacement of diaphragm 69 in the same manner as the electrostatic capacitance C67 and has a harmful effect on the measurement.
FIG. 14 is a schematic showing the electric circuit formed between lead pins A.sub.1, A.sub.2, and A.sub.3. In FIG. 14, the electrostatic capacitance formed between lead pins A1 and A3 is expressed by Co/(1-.DELTA./d)+C67, and the electrostatic capacitance formed between lead pins A2 and A3 is expressed by Co/(1+.DELTA./d)+C68.
The electrostatic capacitances C67 and C68 have a harmful effect on the measurement, because they are typically similar in magnitude to Co. For example, let diaphragm 69 and fixed electrodes 61 and 62 each be 9-mm squares; let air gaps 65 and 66 be 7-mm circles filled with silicone oil; let the glass junctions 67 and 68 be made of SM-36A, a glass bonding material produced by the Nippon Electric Glass Company, Ltd.; let the thickness of air gaps 65 and 66, and glass junction 67 and 68 be 12-mm. The specific inductive capacitances of silicone oil and the glass bonding material SM-36A are 2.65 and 4.8, respectively. The permittivity constant is 8.85.times.10.sup.-14 farads/cm. Then the electrostatic capacitance Co would be 75.21 pF, and C67 and C68 would both be 150.50 pF. Thus, the electrostatic capacitances C67 and C68, which have no relation to the measurement, are about twice as large as the electrostatic capacitance Co.
If the capacitances C67 and C68 were small enough to be negligible in comparison with the capacitance Co, the capacitances C10 and C20 given by C10=Co/(1-.DELTA./d) and C20=Co/(1+.DELTA./d), would vary differentially such that the signal F would be proportional to the diaphragm displacement .DELTA., in accordance with the following equation: EQU F=(C10-C20)/(C10+C20)=.DELTA./d
In the case of the example shown in FIG. 13, however, since each of the capacitances C67 and the C68 are about twice as large as the capacitance Co, the signal F is not expressed by the above equation. Higher order terms occur so that the signal F is no longer linear. One countermeasure uses extremely large electrode areas at the air gaps 65 and 66, in order to make capacitances C67 and C68 small in comparison with the capacitance Co. However, the size of such a pressure detector them becomes extremely large. Therefore, this countermeasure is not preferable.
A twin-diaphragm electrostatic capacitance type pressure detector is disclosed in U.S. Pat. No. 4,169,389 entitled "PRESSURE MEASURING DEVICE", the Assignee of which is Fuji Electric Co., Ltd.
FIG. 15 is a sectional view showing the main portion of a twin-diaphragm electrostatic capacitance type pressure detector. The detector comprises a center electrode 123, diaphragms 127 and 128, and fixed electrodes 121 and 122. Diaphragms 127 and 128 and fixed electrodes 121 and 122 are disposed on opposite sides of, and symmetrically with respect to, center electrode 123.
Center electrode 123 is provided with an electric conductor plate 123a at its outer periphery. Diaphragms 127 and 128 are joined to center electrode 123 by glass junctions 131 and 132. Hollow portions 131h and 132h are formed in glass junctions 131 and 132. Therefore, the gaps between diaphragms 127 and 128 and center electrode 123 are equal to the thicknesses of the glass junctions 131 and 132, respectively.
Fixed electrodes 121 and 122 are joined to the other sides of diaphragms 127 and 128. Air gaps provided between diaphragm 127 and fixed electrode 121, and between diaphragm 128 and fixed electrode 122. Pressure leading-in holes 121h and 122h are formed in fixed electrodes 121 and 122 so as to penetrate their center portions. Electric conductor plates 121a and 122a are provided on the outer peripheral surfaces of electrodes 121 and 122, respectively. Lead pins A1, A2, and A3 are in contact with the electric conductor plates 121a, 122a, and 123a and electrostatic capacitances are taken out through those lead pins.
Pressure P1 acts through pressure leading-in hole 121h of fixed electrode 121 onto a gap U1 on the right side of diaphragm 127. Pressure P2 acts through pressure leading-in hole 122h of fixed electrode 122 and the hollow portions 131h and 132h of glass junctions 131 and 132 onto a gap U2 between diaphragm 127 and center electrode 123, a gap U3 between center electrode 123 and diaphragm 128 and a gap U4 between diaphragm 128 and fixed electrode 122. The pressure difference (P2-P1) therefore does not displace diaphragm 128, while diaphragm 127 is displaced.
The pressure detector shown in FIG. 15 is generally placed within a hermetically sealed housing. Two seal diaphragms receive the pressures P1 and P2. Alternatively, the pressure detector may be placed within a hermetically sealed housing, with a single seal diaphragm, and an incompressible fluid for pressure transmission, such as silicone oil, enclosed in the housing.
The detector shown in FIG. 15 is of the twin-diaphragm type, and has the following features: (1) It can be used even when the media are different from each other. For example, when the gap U1 is filled with medium M and the gaps U2, U3 and U4 are filled with another medium N; (2) it is possible to easily restrain and compensate for the effect of a change in temperature.
FIG. 16 is an equivalent circuit diagram of the electrostatic capacitances in the twin-diaphragm type detector. The electrostatic capacitance between the fixed electrode 121 and the center electrode 123 in FIG. 15, which is taken out through the lead pins A1 and A3, is formed by the parallel connection of (1) an electrostatic capacitance Co/(1-.DELTA./d) formed between the center electrode 123 and the diaphragm 127, and (2) an electrostatic capacitance C131 formed through the respective peripheral portions of the fixed electrode 121, the diaphragm 127 and the center electrode 123.
In the above expression for the electrostatic capacitance Co/(1-.DELTA./d), d represents the gap between the right side of the center electrode 123 and the diaphragm 127 when the differential pressure (P2-P1) acting on the diaphragm 127 is zero. Co represents the electrostatic capacitance at that time, and C131 is the electrostatic capacitance across the thickness of the glass junction 131. It is assumed that the pressure P2 is larger than the pressure P1.
In the same manner, the electrostatic capacitance between the lead pins A2 and A3 is an electrostatic capacitance of a parallel connection of the electrostatic capacitance Co between the left side of the center electrode 123 and the diaphragm 128 and an electrostatic capacitance C132 across the thickness of the glass junction 132.
As has been described above, in the conventional twin-diaphragm type detector, since the capacitance C131 and the capacitance C132 are not so small as to be negligible in comparison with the capacitance Co, it is impossible to obtain a detection signal proportional to the displacement .DELTA. of the diaphragm 127. The differential pressure (P1-P2) cannot be obtained from a signal given by F=(C10-C20), where C10 represents the composite electrostatic capacitance between the lead pins A1 and A3 and C20 represents the composite electrostatic capacitance between the lead pins A2 and A3.